The present invention relates to a method of producing a laminated packaging material comprising a core layer of paper or paperboard and a barrier layer applied on one side of the core layer.
The present invention also relates to a laminated packaging material produced according to the method, as well as to packaging containers which are produced from the laminated packaging material. Particularly advantageous packaging laminates in which polyvinyl alcohol or starch in combination with nanoparticles is used as a barrier layer material are provided.
It is well-known in the packaging industry to employ laminated packaging material of a single-use nature for packing and transporting liquid foods. Normally, such laminated packaging materials are built up from a configurationally rigid but foldable core layer consisting of, for example, paper or paperboard in order to achieve good mechanical configurational stability. Liquid-tight coatings of plastic are applied on both sides of the core layer and effectively protect the core layer of liquid-absorbing fibre from penetration by moisture. These outer layers normally consist of a thermoplastic, preferably polyethylene, which moreover impart to the packaging material superior thermosealing properties, whereby the packaging material may be converted into finished packages with the desired geometric configuration.
However, laminated packaging material consisting solely of paper or paperboard and liquid-tight plastic lacks tightness properties vis-a-vis gases, in particular oxygen gas. This is a major drawback in the packing of many foods whose shelf-life deteriorates dramatically when in contact with oxygen gas, such as for example fruitjuices. In order to supplement the packaging material with a barrier against gases, especially oxygen gas, it is known in the art to apply a layer possessing superior oxygen gas tightness properties, for example aluminum foil or polyvinyl alcohol, on that side of the core layer which is intended to face in towards the inside of the package.
In comparison with aluminum foil, polyvinyl alcohol enjoys many desirable properties, with the result that it is preferred as barrier material in many contexts. Among these, mention might be made of the polyvinyl alcohol""s superior strength properties, compatibility with foods and economic value, together with its excellent oxygen gas barrier properties. Moreover, it has been considered to be expedient, in certain cases from the point of view the environment and recycling, to replace aluminum foil as the gas barrier material in food packages.
Like many other conceivable barrier or adhesive polymers such as, for example, ethylene vinyl alcohol, starch, starch derivate, carboxy methyl cellulose and other cellulose derivates or mixtures thereof, polyvinyl alcohol is suitably applied by means of a coating process, i.e. in the form of a dispersion or aqueous solution which, On application, is spread out to a thin, uniform layer on the substrate and thereafter dried. We have found that one drawback in this process however is that an aqueous polymer dispersion or polymer solution of, for example, polyvinyl alcohol with an addition of EAA which is applied on a core layer of paper or paperboard penetrates into the liquid-absorbing fibres of the core layer. In connection with the removal of water for drying and possibly for curing the applied barrier layer, the core layer is also subjected to elevated temperatures for drying, and as a result the risk of undesirable crack formation in the paperboard or paper layer, respectively, increases as a result of the moisture content which is difficult to adjust, and the drying which takes place in this layer.
Swedish Patent No. 440519 proposed including a thickening agent such as alginate to reduce penetration of water into the board. The use of PVOH as a barrier material applied over a polymer layer preventing crack formation and smoothing the board surface was disclosed in WO97/13639.
One drawback is that the polyvinyl alcohol is moisture sensitive and rapidly loses its barrier properties when it is exposed to a damp environment. This inconvenience was previously obviated according to WO97/22536 by combining the polyvinyl alcohol with one or more per se known food-approved polymers, for example ethylene acrylic acid Copolymer (EAA) or styrene butadiene copolymer. These advantageously form, in combination with polyvinyl alcohol, a coherent, well integrated layer possessing superior gas barrier properties, in particular oxygen gas barrier properties, at the same time as the desired superior gas barrier properties of the polyvinyl alcohol are retained even in a damp environment.
WO97/22536 disclosed that polyvinyl alcohol mixed with EAA-ethylene copolymer or the like material could be dispersion coated onto a paperboard previously coated with a polymer and thereafter could be dried and cured at temperatures of up to 170xc2x0 C. to form a laminated packaging material with a very good barrier property.
Without being restricted to any particular theory, it is suggested that the improved oxygen and water barrier properties results from an esterification reaction between the PVOH and the EAA all the increased curing temperature, whereby the PVOH is crosslinked by hydrophobic EAA polymer chains, which thereby are built into the structure of the PVOH.
Another drawback in the employment of, for example, polyvinyl alcohol as barrier layer instead of aluminum foil is that, on storage of lightsensitive foods, it is necessary in many cases also to incorporate into the packaging material a light barrier of some type. Granted, a core layer of paper or paperboard does not (to the naked eye) allow the passage of any light, but light in invisible wavelength ranges nevertheless penetrates through from the outside of a packaging container to the packed food product and may have a negative effect on it from the point of view of shelflife. The employment of aluminum foil in the packaging material enjoys that advantage that the aluminum foil in itself constitutes a good barrier against both gases and against light. On the other hand, polyvinyl alcohol is as good as completely transparent even in mixtures with a hydrophobic polymer such as ethylene acrylic acid copolymer or styrene butadiene copolymer. The admixture of conventional light barriers, such as carbon black and titanium dioxide into any of the plastic layers included in the laminated packaging material according to WO97/22536 is per se possible, but would entail an aesthetically unattractive appearance in the package.
Yet a further drawback inherent in the laminated packaging material including barrier layers of, for example, polyvinyl alcohol possibly together with another polymer is that this packaging material cannot be produced employing the same production equipment as in the production of packaging material using aluminum foil as the barrier layer, which involves capital investment costs for new production equipment.
As indicated above, PVOH has environmental benefits as a barrier material. In addition to such synthetic materials, the possibility of using natural and biodegradable polymers (biopolymers) such as starch and starch derivatives, as gas barrier materials has been investigated.
It is previously known that starch possesses some gas barrier properties when employed in relatively thick layers, such as in films having a thickness of about 20 to 30 xcexcm. Such thick layers of starch material are not suitable for use in packaging laminates however, since they become brittle and are prone to cracking and breaking upon handling, for example in the lamination process and when fold forming of the laminate into packages. Besides not being flexible in handling at manufacturing and distribution, laminates including such thick layers of starch may also absorb moisture and cause delamination between the starch layer and its adjacent layers.
From WO97/16312 it is known that very thin layers of starch applied on to a core layer may provide some gas barrier properties, at least when employed together with an adjacent layer of plastics, which has been united with starch barrier layer by extrusion coating of the plastics material. Two very thin layers of starch, applied in a quantity of 0.5 and 1 g/m2 respectively, dry weight, on to opposite sides of a core layer of paperboard and each extrusion coated with a layer of plastics, provided an oxygen gas barrier of 289 cm3/m2, per 24 h at 1 atm. Similarly, two layers of starch, applied in a quantity of 1 and 1.5 g/m2 respectively, provided an oxygen gas barrier of 141 cm3/m2, per 24 h at 1 atm. The results obtained were thus, comparable with the gas barrier properties of, for example, a 12 xcexcm thick film of oriented PET, thus representing a xe2x80x98medium performance barrierxe2x80x99 material.
The packaging laminate WO97/16312 is, however, merely a medium performance gas barrier material. This means that it may only be used for packaging of liquid food products during short time periods of cool storage. It is not hitherto known in the prior art to produce packaging laminates having high performance gas barrier properties from starch of or starch derivative barrier materials. It would be much more desirable to be able to provide packaging material having sufficient gas barrier properties for long time storage of liquid food products, i.e. for extended shelf life (ESL) at cool storage or even for aseptic storage. Such desirable high performance oxygen gas barrier properties are in the order of about 50 cm3/m2 at 24 h, 1 atm (23xc2x0 C., 50% RH) or better, e.g. up to 30 cm3(m2 at 24 h. 1 atm, i.e. oxygen gas barrier properties comparable to those of, for example, PVOH, EVOH (ethylene vinyl alcohol copolymer) or polyamides (PA) when employed at a thickness of the order of about 5 xcexcm.
FR-A-2684922 discloses coating a polymer film such as polyester with a dispersion of amylose starch containing surfactant and drying the starch at a temperature of up to 180xc2x0 C. Good gas barrier properties are obtained at coating levels of for instance 0.7 g(dry)/m2. However, there is no indication that similar properties might be obtainable in a laminated packaging material having a paper or paperboard core.
However, although the above gas barrier polymer materials are capable of providing good gas barrier properties in a packaging laminate they are still oxygen-permeable to some degree, while a metal or glass material to be used in canning or bottling has an oxygen permeability of substantially zero. In order to improve the gas barrier properties still further, the polymer gas barrier material may be mixed with an inorganic laminar compound. Such a gas barrier resin composition is for example described in EP-A-590263, wherein excellent high level gas and moisture barrier properties are obtained. EP-A-590263 discloses a process for producing a gas barrier resin composition or its moulded article including a film, the composition comprising a resin and an inorganic laminar compound having a particle size of 5 having a particle size of 5 m or less and an aspect ratio of 50 to 5000, the process comprising dispersing the inorganic laminar compound in a resin or resin solution in the state that the inorganic laminar compound is swollen or cloven with a solvent/dispersant and removing the solvent from the dispersion, if necessary in the form of a film, while keeping the laminar compound in the swollen state.
We have now found that a laminated packaging material possessing excellent to barrier properties, in particular against gases, may be produced using a method which lends itself to being carried out using conventional production equipment of the type employed in the production of packaging materials with aluminum foil as the barrier layer.
We have also now established that it is possible in a paperboard packaging laminate for liquid food packaging to obtain excellent high performance oxygen barrier properties from the use of a gas barrier composition comprising a dispersible or soluble polymer and an inorganic laminar compound.
Furthermore, by avoiding coating the liquid gas barrier composition onto the core layer in connection with the lamination of the packaging material, we have eliminated the risk of excessive water absorption into the core layer and consequential crack formation when drying the coated core layer of paper or paperboard.
According to a first aspect of the invention, there is now provided a method of producing a laminated packaging material comprising a core layer of paper or paperboard and a barrier layer applied on one side of the core layer, characterised in that a liquid gas barrier resin composition including a dispersion or solution of a polymer and an inorganic laminar compound is applied as a barrier layer on at least one side of a carrier layer and is dried during heating for driving off the dispersant or solvent, whereafter the carrier layer with the applied, dried barrier layer is combined and permanently united with one side of the core layer.
Preferably, the inorganic laminar compound or so-called nanoparticle compound is dispersed to an exfoliated state, i.e. the lamellae of the layered inorganic compound are separated from each other by means of a liquid medium. Thus the layered compound preferably may be swollen or cloven by the polymer dispersion or solution, which at dispersion has penetrated the layered structure of the inorganic material. It may also be swollen by a solvent before added to the polymer solution or polymer dispersion. Thus, the inorganic laminar compound is dispersed to a delaminated state in the liquid gas barrier composition and in the dried barrier layer.
The term clay minerals includes minerals of the kaolinite, antigorite, smectite, vermiculite or mica type, respectively. Specifically, laponite, kaolinite, dickite, nacrite, halloysite, antigorite, chrysotile, pyrophyllite, montmorillonite, hectorite, sodium tetrasilicic mica, sodium taeniolite, commonmica, margarite, vermiculite, phlogopite, xanthophyllite and the like may be mentioned as suitable clay minerals.
The inorganic laminar compound or clay mineral preferably has an aspect ratio of 50-5000 and a particle size of up to about 5 xcexcm in the exfoliated state.
Preferably, the barrier layer is applied by means of liquid film coating with an aqueous composition of a dispersion or solution of a barrier polymer further including the inorganic laminar compound. For example PVOH, or PVOH and EAA, may be applied in the state of an aqueous solution in mixture with an inorganic laminar compound, whilst starch may be applied in an aqueous partially dispersed and/or dissolved state in mixture with the inorganic laminar compound.
Preferably, the barrier layer includes from about 1 to about 40 weight %, more preferably from about 1 to about 30. weight % and most preferably from about 5 to about 20 weight %, of the inorganic laminar compound based on dry coating weight. If the amount is too low, the gas barrier properties of the coated and dried barrier layer will not be markedly improved compared to when no inorganic laminar compound is used. If the amount is too high, the liquid composition will become more difficult to apply as a coating and more difficult to handle in storage tanks and conduits of the applicator system.
Preferably, the barrier layer includes from about 99 to about 60 weight %, more preferably from about 99 to about 70 weight % and most preferably from about 95 to about 80 weight % of the polymer based on the dry coating weight.
An additive, such as a dispersion stabiliser or the like, may be included in the gas barrier composition, preferably in an amount of not more than about 1 weight % based on the dry coating.
The barrier layer is preferably applied on the carrier layer in an amount, depending on the kind of polymer, of approximately 0.5 to 20 g/m2, more preferably approximately 1-10 g/m2, based on dry weight. If the coated amount is too low, the gas barrier properties may be inferior, while if the amount is too high, there is a risk for an inflexible barrier layer and crack formation therein.
The polymer preferably is a high hydrogen-bonding polymer having hydrogen-bonding groups or ionic groups to an extent of 20 weight % and above of the polymer molecule. More preferably, the polymer has functional hydroxyl groups and may for instance be selected from among polyvinyl alcohol (PVOH), ethylene vinyl alcohol (EVOH), polysaccharides such as starch, starch derivatives, carboxyl methyl cellulose and other cellulose derivatives, or a mixture of two or more thereof. Also polymers having nitrogen-containing groups may be employed. Most preferably, the polymer is a polymer having gas barrier properties itself, specifically polyvinyl alcohol, starch or a starch derivative.
Said aqueous polymer dispersion or polymer solution applied as barrier layer may be dried and optionally cured at a web surface temperature of approximately 80 to 200xc2x0 C. For non-curing materials it is preferred to operate at a temperature of approximately 80 to 1300xc2x0 C.
Most preferably, materials including PVOH and inorganic laminar compound are preferably first dried at web temperatures from 80 to 160xc2x0 C. (preferably 140 to 1600xc2x0 C.) in a first step and are cured at web temperatures from 170 to 230xc2x0 C. in a second stage resulting in an improved gas barrier at 80% RH. Optionally, the carrier and barrier material may be cooled between the two steps.
A polymer with functional carboxylic acid groups may also be included. This may react with the polymer with functional hydroxy groups during the drying/curing process.
Suitably, the polymer with functional carboxylic acid groups is selected from among ethylene acrylic copolymer and ethylene methacrylic acid copolymers or mixtures thereof.
One particularly preferred barrier layer mixture is of polyvinyl alcohol, ethylene acrylic acid copolymer (EAA) and an inorganic laminar compound. The EAA copolymer is preferably included in the barrier layer in an amount of about 1-20 weight %, based on dry coating weight.
Another particularly preferred barrier layer mixture is of starch or starch derivative and an inorganic laminar compound.
Optionally, the barrier layer is first dried and is then heated to a higher temperature so that the dried barrier layer is cured at a temperature of up to 230xc2x0 C. preferably approximately about 170xc2x0 C. The high temperature curing may be of short duration, such as corresponding to web speeds normally used in the packaging laminate production.
The carrier layer may consist of paper or plastics or plastics coated paper and preferred materials are described below. When paper is employed it is preferably thin. In one option the carrier layer preferably consists of paper with a grammage of approximately 5-35 c/m2, e.g. 7-25 g/m2, more preferably approx. 10-20 g/m2.
The carrier layer bearing the barrier material and the core layer may be assembled together in various ways.
The carrier layer bearing at least one said barrier layer may be combined and united with the core layer by extrusion of a layer of thermoplastics therebetween.
Where said carrier layer bears a said barrier layer on one side thereof it therefore may be combined with the core layer by extrusion of a layer of thermoplastics between the carrier layer and the core layer.
An outer layer of thermoplastics, preferably polyethylene, is then applied on the barrier layer by means of extrusion.
When the carrier layer bears a said barrier layer on one or both sides it may be combined with the core layer by extrusion of a layer of thermoplastics between the core layer and a said barrier layer.
If said carrier layer bears a said barrier layer on both sides thereof, a layer of thermoplastics may then be applied to the outer layer of barrier material by extrusion.
The layer of plastics applied between the core layer and the carrier layer or a said barrier layer may include a substance functioning as light barrier. This is especially preferred when the carrier layer is of paper or other visually non-transparent material.
We have also established that it is possible in a packaging laminate to obtain high performance oxygen barrier properties from the use of a gas barrier composition comprising a dispersion of starch and similar materials and an inorganic laminar compound.
A gas barrier layer including starch and inorganic laminar compound is preferably applied at a dry coating weight of from 0.5 to 5 g/m2, more preferably 0.5 to 3 g/m2 e.g. from 1.5 to 2 g/m2.
It is acceptable to include minor amounts of other polymeric materials which do not interfere with the desired properties of the composition of starch and inorganic laminar compound. For instance the gas barrier layer may further comprise a minor amount of water soluble or water dispersible polymers having functional hydroxyl groups, e.g. polyvinyl alcohol, and carboxyl group containing polyolefins such as ethylene acrylic acid, or a mixture thereof. The amount of such materials may be from 0 to 30%, e.g. 0 to 20% or 0 to 10% by weight.
Preferably, packaging laminates including starch in the barrier layer comprises a layer of plastics polymer, preferably a thermoplastics, e.g. polyethylene, laminated directly with the said gas barrier layer. Most preferably, said polymer is LDPE. Other thermoplastics that may be employed include all other kinds of polyethylene (including LLDPE, ULDPE, VLDPE, M-PE and HDPE), polyproplylene, and polyethyleneterephthalate.
We have observed that when polyethylene is applied to a layer based on starch at a high temperature, e.g. over 200xc2x0 C., the gas barrier properties are improved and that under appropriate conditions can be made to reach or move further into a high performance level. According to the invention, the preferred method of obtaining optimal properties is to apply the barrier composition based on starch or starch derivative not to a thick core layer as in WO97/16312 but to a separate carrier. Suitably then, the gas barrier layer is carried by a carrier layer of paper or plastics.
When paper is employed it is preferably thin, e.g. said carrier layer may be of paper having a surface weight of from 5 to 35 g/m2, preferably from 10 to 25 g/m2. The paper may also be coated beforehand with a layer of plastics.
After application of the liquid composition of starch and inorganic laminar compound, the carrier may be combined with a thicker core material so that the packaging laminate comprises a core layer having said carrier layer on one surface side thereof. There may be one or more layers including a heat sealing layer on the other surface side of said core layer.
The surface of the carrier layer to which the starch or starch derivative composition is applied is preferably substantially impervious to said liquid vehicle.
The degree to which the surface is impervious may be measured by measuring surface adsorption, e.g. in Cobb units. (xe2x80x9cCobbxe2x80x9d=g(water)/m2 adsorbed on to the surface in 60 seconds exposure to liquid water). Adsorption of other liquids could be measured in an analogous method. The method of measuring Cobb adsorption is defined in SCAN P12-64 and in TAPPI T441. The surface adsorption of plastics is generally about 1 Cobb, whilst a smooth paper surface will generally have an adsorption of about 20 to 30 Cobb. Suitably, for use in the invention the substrate surface should have an adsorption of 50 Cobb or less, preferably an adsorption of 30 Cobb or less, more preferably an adsorption of less than 20 Cobb or most preferably an adsorption of 10 Cobb or less, e.g. less than 5 Cobb.
Preferably, the surface of the carrier layer to which the composition of polymer and inorganic laminar compound is applied has a smoothness of 200 Bendtsen or better. The method of measuring Bendtsen smoothness is defined in SCAN (Scandinavian Pulp and Paper Norms) P21-67 and in TAPPI UM535.
Where the substrate is plastics or has a plastics surface, such desired smoothness is usually obtained, such as in, for example, a film of plastics or a plastics coated paper carrier layer.
One reason why a high performance barrier property was not achieved in WO97/16312 may be that the paperboard core layer lacked the requisite degree of impermeability so that the aqueous solution of starch which was employed may have penetrated the surface. This might have an adverse action in a number of ways. There may not then be a smooth and unbroken surface to the starch layer because of penetration as such into the paperboard. Alternatively, or additionally, drying of the paperboard to dry the starch layer may cause surface deformation of the paperboard and hence cracking of the starch layer. These problems are obviated when the starch is applied to a separate, smooth, impervious carrier layer which is subsequently laminated to the core layer.
The paperboard used in WO97/16312 would typically be expected to have had a surface smoothness of 500-600 Bendtsen. This may in itself have been sufficient to prevent the starch layer being smooth and unbroken or from having thin areas providing a path for oxygen transmission.
In order to avoid cracks, punctures or deformations in the barrier composition layer of starch or starch derivative layer and inorganic laminar compound, it is preferred that the surface on to which it is applied is smooth, e.g. that the substrate surface has a smoothness of 200 Bendtsen or better (i.e. less), e.g. from up to 150 Bendtsen, most preferably up to about 100 Bendtsen.
The materials described as carrier for use with starch can also be used with the other barrier materials used according to the first aspect of the invention. However, generally a plastics film carrier or a plastics coated thin paper carrier is preferred when using starch and the use of a thin paper or a plastics coated thin paper carrier is preferred for barrier materials such as PVOH which may be heated to temperatures well above 100xc2x0 C. for drying and curing.
Starch for use in the invention may be of any conventional type although certain starches provide better results than others under the conditions we have used. Modified potato starch is preferred, such as Raisamyl 306 (Raisio) which is hypochlorite oxidised. Other acceptable starches include corn starch and derivatives, such as Cerestar 05773 a hydroxypropylated corn starch.
Starch derivatives that are suitable for use in the invention include oxidised starch, cationic starch and hydroxpropylated starch.
It will be understood that when the gas barrier property of the packaging laminates of the invention is referred to as being provided by a particular material, e.g. a composition of starch or a starch derivative and an inorganic laminar compound, this does not exclude the case where the gas barrier property is the result of an interaction between the stated material and an adjacent layer in the laminate, rather than a bulk property of the stated material viewed in isolation.
It may be that a contributing mechanism in the improvement in barrier property noted when polyethylene is applied at a high temperature to a layer of starch comes from penetration of polyethylene molecules into the starch, replacing water in starch crystals. Other polymers producing a similar effect may be used.
Said plastics layer may be applied to said composition of starch or starch derivative and inorganic laminar compound by melt extrusion or may be applied as a preformed film by hot pressure lamination e.g. with a heated roller. Generally, any technique may be employed in accordance with this preferred embodiment that provides the required modification of the barrier property of the starch.
Preferably said plastics layer is bonded to the layer of starch or starch derivative and inorganic laminar compound at a temperature of at least 200xc2x0 C., preferably from 250 to 350xc2x0 C. most preferably from 250 to 330xc2x0 C.
According to a second aspect of the invention, a laminated packaging material is provided, which is produced according to the method of the invention.
According to a third aspect of the invention, a packaging container is produced by fold formation of a sheet or web-shaped laminated packaging material obtained by the method according to the invention.
By applying, in a separate production stage, a liquid composition of a polymer dispersion or polymer solution and an inorganic laminar compound as a barrier layer on at least one side of a carrier layer and drying the barrier layer during heating for driving off the liquid medium, preferably water, and thereafter combining and permanently uniting the carrier layer with the applied, dried barrier layer to one side of the core layer, there will be realised a laminate packaging material with a barrier layer possessing superior barrier properties.
Thanks to the fact that the barrier layer is not dried or cured at elevated temperature in connection with the lamination of the packaging material, the risk of excessive water absorption into the core layer and of drying of the core layer of paper or paperboardxe2x80x94with consequential risk of crack formation in the core layerxe2x80x94is wholly eliminated.
Given that the plastics layer applied between the core layer and a paper carrier layer may include a substance serving as light barrier, ideally carbon black, a light barrier layer will be realised whose unattractive black appearance may be concealed in a layer between the core layer and a thin paper layer carrying the barrier layer.
One important advantage of the method according to the aspect of the present invention is that the barrier layer produced in a separate stage may be employed in the production of a laminated packaging material in a corresponding manner and using corresponding production equipment as are employed today in the production of packaging materials with aluminum foil as the oxygen gas barrier.